Tropical Forests -- Earth's Air Conditioner

Planting and protecting trees--which trap and absorb carbon dioxide as they grow--can help to remove carbon dioxide from the atmosphere. But a new study suggests that, as a way to fight global warming, the effectiveness of this strategy depends heavily on where these trees are planted. In particular, tropical forests are very efficient at keeping the Earth at a happy, healthy temperature.

The researchers, including Ken Caldeira of Carnegie's Department of Global Ecology and Govindasamy Bala at Lawrence Livermore National Laboratory, found that because tropical forests store large amounts of carbon and produce reflective clouds, they are especially good at cooling the planet. In contrast, forests in snowy areas can warm the Earth, because their dark canopy absorbs sunlight that would otherwise be reflected back to space by a bright white covering of snow.

The work simulates the effects of large-scale deforestation, and accounts for the positive and negative climate effects of tree cover at different latitudes. The result, which appears in this week's early online edition of the Proceedings of the National Academy of Sciences, makes a strong case for protecting and restoring tropical forests.

"Tropical forests are like Earth's air conditioner," Caldeira said. "When it comes to rehabilitating forests to fight global warming, carbon dioxide might be only half of the story; we also have to account for whether they help to reflect sunlight by producing clouds, or help to absorb it by shading snowy tundra."

Forests in colder, sub-polar latitudes evaporate less water and are less effective at producing clouds. As a result, the main climate effect of these forests is to increase the absorption of sunlight, which can overwhelm the cooling effect of carbon storage.

However, Caldeira believes it would be counterproductive to cut down forests in snowy areas, even if it could help to combat global warming. "A primary reason we are trying to slow global warming is to protect nature," he explains. "It just makes no sense to destroy natural ecosystems in the name of saving natural ecosystems."